The Daily Diet Score. Chapter 3 BODY MASS INDEX: HEY, WHO ARE YOU CALLING FAT? Mark A. Carlson, MD

Transcription

1 The Daily Diet Score Chapter 3 BODY MASS INDEX: HEY, WHO ARE YOU CALLING FAT? Mark A. Carlson, MD 1

2 Chapter 3 Most of us don t need a scientific formula to know if we are fat. That information usually comes from looking in the mirror, struggling to button a pair of pants, seeing our own picture, etc. The only number that may be important to us in this respect is the number on the bathroom scale. And on an individual basis, this approach usually is OK. Most of us have a notion of what our ideal weight is. But a problem arises when scientists, nutritionists, and other concerned professionals try to compare dietary treatments. On its own, body weight has been inadequate measure for comparative purposes. This is where a measurement called the body mass index (BMI) comes in. Although definitely not the best fatness measurement available, the BMI probably is the most popular. And this is why the BMI is relevant to the dieter, and why the BMI is being discussed early on in the Daily Diet Score. In this chapter, I will describe how body size can be quantified with the BMI. Classifying an individual only by weight can be misleading because of the influence of height. For example, take two men who each weigh 200 pounds. The first man is six foot six inches tall, and the second man is five foot four inches tall. It does not take much imagination to appreciate that the 6 6 man is not overweight, but the 5 4 man is. It may not sound fair, but the shorter man cannot carry the same amount of weight as the taller man without being fat. Naturally there are some exceptions to this simple observation. A woman eight months pregnant may not be overweight, even though she weighs 30 pounds more than she did before the pregnancy. As another example, a powerlifter who is 5 4 and weighs 200 pounds may be packed full of muscle and bone and have very little (less than 10%) body fat. But these are the exceptions to the rule. For the rest of us who are not powerlifters, being 5 4 and weighing 200 pounds usually means that we are fat. 2

3 The Daily Diet Score In the real world, a 6 6 man who weighs 200 pounds may be OK, but pork him up to 300 pounds now he is obese. So the relationship between height and weight is very important. How can this relationship described? With the Body Mass Index, or BMI. The BMI is a number which, in most situations, describes the relative amount of fat carried on the frame. The BMI is calculated from a person s height and weight, so the BMI is a more accurate indicator of how fat someone is than their weight alone. In other words, the BMI is a description of your body size based on your height and weight. How do you figure out your BMI? Probably the easiest way is if you have internet access. If you performed a search of body mass index, a number of web sites with BMI calculators would be on the hit list. For example, check out the BMI calculator provided by the National Center for Chronic Disease Prevention and Health Promotion. All you have to do on this site is enter your height and weight, click on Calculate, and you have your BMI. For those of you who are mathematically inclined, you can calculate your BMI with the following formula: BMI = [(weight in pounds) (height inches) 2 ] 703 Alternatively, you can read your BMI from a table, which I have included with this chapter (see Table 3-1; courtesy of the National Heart, Lung and Blood Institute). Looking at our previous male examples, the 5 4 /200 lb guy has a BMI of about 34.5, while the 6 6 /200 lb guy has a BMI of 23 (Table 3-1 only goes up to 6 4, so I used an internet calculator for the taller 3

4 Chapter 3 man). For comparison, the 6 6 /300 lb gentleman has a BMI of 34.7 very close to the 5 4 /200 lb subject. Incidentally, the BMI calculation is done the same for women as it is for men. That is, a 5 4 /200 lb woman also will have a BMI of This one-size-fits all approach of the BMI to both sexes has been a source of criticism, which I will discuss in a later chapter. So what do these BMI values mean? There are a variety of classification schemes for BMI. The National Heart, Lung, and Blood Institute (part of the National Institutes of Health, or NIH) has divided BMI values into four categories, as shown below and in Table 3-1: normal overweight obese 40 extreme or morbid obesity There are other BMI categories, too: is underweight; and less than 17 is anorexic. The implications of these low BMI categories will be covered in a later chapter. Recently, the super obese category was added for those individuals with a BMI of greater than 50. For example, a 5 4 person who has weighs 300 pounds will have a BMI of The super obese category is of concern to obesity surgeons, who may treat patients with BMIs greater than 60 or 70. A 5 4 person would have to weigh 408 pounds to have a BMI of 70. According to the American Obesity Association, about 60% of Americans have a BMI of 25 or greater (i.e., they are at least overweight). About half of this group, or about 30% of all Americans, have a BMI of 30 or greater. So over 60 million adults in this country are obese (!). 4

5 The Daily Diet Score You might be dismayed about how little wiggle room the BMI system gives you before you get labeled as overweight. For instance, a 5 2 person who weighs 131 pounds already has a BMI of 24. If this person gains another five pounds, then the BMI increases to 25 which puts this individual into the overweight zone. A 5 10 person who tips the scales at 167 pounds also has a BMI of 24; seven more pounds pushes her/his BMI up to 25. At the other end of the spectrum, the 5 2 person can weigh as little as 104 lb before falling into the underweight category (BMI < 19). Similarly, the 5 10 individual can weigh as little as 132 lb and still be in the normal category for some, 132 pounds may sound way too little for a five-foot ten-inch frame. On the surface, the BMI may appear to be a neat little formula which will tell you whether your fat or not. But as hinted above, we can play around with the BMI and get misleading results. Here s another example: using an internet BMI calculator, a 5 4 woman who weighs 145 pounds will have a BMI of According to Table 3-1, her BMI is still in the normal category that is, she is not fat. But, if she gains a single pound (or worse, stands funny on the scale) and weighs 146 pounds, her BMI is boosted to In this brief moment she has become overweight. Now how is that fair? This little statistical blooper can be present in huge collections of data in which a cutoff number is sought. Take the example of blood pressure readings in adult men. At some point, someone or somebody is going to have to define a blood pressure value where normal ends and high blood pressure begins. The latter usually means having to take blood pressure pills. In our situation, the cutoff point is where normal weight ends and overweight begins. The definition of these transition points usually is based on good scientific data. But when it comes right down to it, the decision of who to call normal and who to call fat is arbitrary. 5

6 Chapter 3 For the poor woman above, why couldn t the overweight cutoff point have been placed at a BMI of 25.1? Then she wouldn t be fat, at least by her BMI. Another problem with the BMI is that it does not take into account the proportions of muscle, bone, and fat. Above I described the potential disconnect in weight vs. height for professional athletes. As might be expected, the BMI is not too helpful in these situations. If our 5 4 /145 lb lady was a shot-putter with 8% body fat, then calculating her BMI would be a waste of time. Such a woman would be packed with muscle, and could not be called fat, even though her BMI would say so. The BMI also does not take into account how body fat is distributed. Let s take two examples of 5 4 women who both weigh 157 pounds. Both have a BMI of 27 (the overweight category). The first woman happens to carry most of her extra weight in her thighs, hips, and buttocks, but has a smallish waist. The second woman carries most of her extra weight in the abdomen (i.e., she has truncal obesity). The first woman has a pear-shaped body, and the second woman is apple-shaped. Such shape differences actually have been recognized as having important health consequences (to be discussed). Essentially, pears are healthier than apples. So the apple-shaped woman carries higher heath risks than the pear-shaped woman. This, despite the fact that both women are overweight with the same BMI. Now how is that fair? It s not fair because the BMI is not a perfect descriptor of excessive body fat. There are other ways to describe how weight is distributed on the body, and there are more accurate methods to measure body fat (described in a later chapter). Currently the BMI does not work such concepts into its formula. Remember this: the BMI is an example of scientists trying to 6

7 The Daily Diet Score impose order on a human characteristic. Such behavior can create an artificial situation, which can lead to inaccuracies. Despite these drawbacks, the BMI has gained wide usage, and provides a common language for doctors, researchers, and patients to discuss weight issues. The BMI may not be the best descriptor for body fat that we have, but it s what we have right now. It would be to your advantage to gain an understanding of the BMI. So are you overweight? Are you obese, morbidly obese, or even super obese? A reasonably good system, the BMI, is available to answer this question. So calculate your BMI. This will give you an objective measurement of how overweight you are. Moreover, I will use the BMI measurement frequently as I discuss the various concepts of the Daily Diet Score. Which reminds me: by applying the DDS, you will take the pounds off and watch your BMI descend into the normal category. The DDS will provide you with a method of fine-tuning your progress into the normal BMI category. And once you have entered that category, the DDS will keep your BMI in check, and maintain a normal weight. If you have found all the above ramblings on the BMI completely confusing, do not fret or despair. Just keep moving on through these chapters. Make no mistake, the BMI classification system is important. But there will be plenty of opportunities to use and understand it as we progress through the Daily Diet Score. 7

8 Body Mass Index Table Normal Overweight Obese Extreme Obesity BMI Height (inches) Body Weight (pounds) Source: Adapted from Clinical Guidelines on the Identification, Evaluation, and Treatment of Overweight and Obesity in Adults: The Evidence Report.